extension PureXML.Emitting.Options {
    /// These options with an `xsl:output`'s settings layered over them. Only an
    /// explicit setting overrides; unspecified `xsl:output` attributes leave the
    /// caller's corresponding option untouched.
    func applying(_ output: PureXML.XSLT.Output) -> Self {
        var copy = self
        if let indent = output.indent {
            copy.prettyPrint = indent
            // Xalan-style indentation: children on their own lines with no
            // leading spaces (indent-amount 0), the conformance golds' form.
            copy.indent = ""
        }
        if let omit = output.omitXMLDeclaration {
            copy.includeXMLDeclaration = !omit
        }
        if let encoding = output.encoding {
            copy.encodingName = encoding
        }
        if let version = output.version {
            copy.xmlVersion = version
        }
        if let standalone = output.standalone {
            copy.standalone = standalone
        }
        return copy
    }
}